Seat pads for vehicles such as automobiles have been generally formed of flexible or semirigid polyurethane foam segments. For the purpose of improving an occupant's comfort when seated while ensuring support stability and safety and for the purpose of reducing the feeling of fatigue due to a long-period of seating, cushioning properties, hardness, and vibration-absorbing properties have been improved by modifying the material or the structure.
As shown in FIG. 2, such a seat pad mainly includes an under-buttocks section 2, an under-thigh section 3, which are disposed at a seating portion, and side bulging sections 4 on both sides of the seating portion. The under-buttocks section 2 includes a base subsection 2A adjacent to the under-thigh section 3 and a rear-end subsection 2B.
Conventionally, such a seat pad 1 for a vehicle is generally produced by mixing a mixture containing a polyol component, a catalyst, a foaming agent, and other additives in predetermined proportions (hereinafter referred to as “polyol-containing solution”) with an isocyanate component immediately before molding to prepare a urethane stock solution, and then injecting the resulting urethane stock solution into a mold to perform foam molding.
In the seat pad for a vehicle, the under-buttocks section 2 supports most of occupant's weight. The material constituting the under-buttocks section 2 has a close relationship to sitting comfort and riding comfort, and is thus important from the standpoint of the sitting comfort and riding comfort. However, the load due to body weight on portions other than the under-buttocks section 2 is low, and thus the level of importance of these portions is relatively low.
In a known seat pad for a vehicle, in some cases, the side bulging sections 4, the under-thigh section 3, and the like are partially composed of different materials. However, in general, the entire seat pad is composed of the same material. That is, the relatively less important portions other than the under-buttocks section 2 are composed of the same material as that used for the under-buttocks section 2, which is important from the standpoint of the sitting comfort and the riding comfort. Therefore, this causes an increase in weight and cost.
A reduction in vehicle weight for achieving higher fuel-efficiency is an important issue for the automobile industry. Furthermore, lower cost is always desired.
To achieve a reduction in the weight of the seat pad, it is proposed that the portions other than the under-buttocks section of the seating portion are formed of a polyurethane foam segment having lower density and higher impact resilience, compared with those of a polyurethane foam segment constituting the under-buttocks section. In such a seat pad, the portions other than the under-buttocks section are composed of a material having low-density, thereby achieving a reduction in weight. The low-density material has high impact resilience, thereby improving sitting comfort and durability.
However, such a polyurethane form having high impact resilience and low density is expensive. Thus, constituting the portions other than the under-buttocks section with the polyurethane foam segment having low density and high impact resilience has a cost disadvantage.
In the seat pad for a vehicle, load on the portions other than the under-buttocks section is low. Thus, the portions other than the under-buttocks section are not always required to be composed of a material having high impact resilience from the viewpoint of riding comfort and durability.
As a seat pad for a vehicle achieving reductions in weight and cost without deterioration of sitting comfort, riding comfort, and durability, the present inventor proposed a seat pad in which the portions other than the under-buttocks section of the seating portion are partly or wholly composed of a material having lower density and substantially the same hardness, compared with a material constituting the under-buttocks section (see Japanese Unexamined Patent Application Publication No. 2002-353102). As described above, load on the portions other than the under-buttocks section is low. Therefore, when the material constituting the portions other than the under-buttocks section has substantially the same hardness as that of the material constituting the under-buttocks section, even if the portions other than the under-buttocks section are composed of a low-density and inexpensive material, satisfactory performance can be obtained, from the viewpoint of the sitting comfort, the riding comfort, and the durability.
Conventionally, a method for molding a one-piece polyurethane foam product by injecting at least two urethane stock solutions into a mold at the same time or with time differences has been performed. If a seat pad for a vehicle including portions having different densities and substantially the same hardness disclosed in Japanese Unexamined Patent Application Publication No. 2002-353102 can be molded in one piece, productivity can be significantly improved, thus reducing production cost.
Conventionally, such a molded polyurethane foam product including portions having different densities and substantially the same hardness has not been molded in one piece. A polyurethane foam product partly having different physical characteristics, in particular, a molded polyurethane foam product including a molded section having a desired density, as designed, at a desired position, has not been molded in one piece.
Conventionally, when one-piece molding has been performed with at least two urethane stock solutions having different compositions, the one-piece molding has been performed by injecting the urethane stock solutions into a mold without consideration of molding pressure during one-piece molding (maximum internal pressure reached in a mold during foam molding). Thus, even when one-piece molding was performed by injecting at least two urethane stock solutions capable of forming polyurethane foam segments having different physical characteristics into a mold under a predetermined pressure P, it was not possible to form portions having target physical characteristics.